CreditOptimized

/*

    This is program to check if credit card numbers are valid using Luhns algorithm and simple rules
    for the starting and lenght numbers of cards
    Optimized version

*/


#include <cs50.h>
#include <stdio.h>


long mypow(int base, int power);
int countdigits(long number);
string ccchecker(long c_card);

int main(void)
{
    long c_card = get_long("Number: ");
    string result = ccchecker(c_card);
    printf("%s\n", result);
}

string ccchecker(long c_card)
{
    //First we use the simple rules to check if a credit card is valid or not
    //First we use a function to get the total ammount of digits and we create a variable to store the card type
    int t_digits = countdigits(c_card);
    string c_brand;
    //Part of the rules are the first digits so we use my exponiation function to get those
    int firstdigits = c_card / (mypow(10, (t_digits - 2)));

    if ((t_digits == 13 || t_digits == 16) && ((firstdigits / 10) == 4))
    {
        c_brand = "VISA";
    }
    else if (t_digits == 15 && (firstdigits == 34 || firstdigits == 37))
    {
        c_brand = "AMEX";
    }
    else if (t_digits == 16 && ((firstdigits > 50 && firstdigits < 56) || firstdigits == 22))
    {
        c_brand = "MASTERCARD";
    }
    else
    {
        //Since the card is invalid we just return that
        return "INVALID";
    }

    //After that we will use the Luhn Algorithm to check if a card has a valid checksum
    //First we create a variable to temporaily store the cc number
    long cc_temp = c_card;
    //then we create a switch that will determine if we should sum or multiply and sum taht will change every other loop
    short simplesumswitch = 1;
    //Variables that we will use
    int digitssum = 0;
    int digitssumsimple = 0;
    int c_digit;

    while (t_digits > 0)
    {
        //now we fork depending on the switch
        if (simplesumswitch == 1)
        {
            //Here we go from the last digit and sum every other digit
            c_digit = cc_temp % 10;
            digitssumsimple += c_digit;
            //Here we are chopping the last digit off
            cc_temp = cc_temp / 10;
            simplesumswitch = 0;
            t_digits--;
        }
        else if (simplesumswitch == 0)
        {
            //Now we go from second to last number and multuiply by 2 every other digit
            //Then we must summ all digits of that multiplication (so if multiplication ends in two digits we sum those)
            c_digit = cc_temp % 10;
            int templuhnsum = c_digit * 2;
            //If a number is 10 or more this will sum both digits
            digitssum += (templuhnsum % 10) + (templuhnsum / 10);
            cc_temp = cc_temp / 10;
            simplesumswitch = 1;
            t_digits--;
        }
    }
    //after the sum we now sum both parts
    int totalsumdigits = digitssum + digitssumsimple;
    //If the last digit of that sum is 0 then the checksum is correct and we return the card type
    if ((totalsumdigits % 10) == 0)
    {
        return c_brand;
    }
    else
    {
        return "INVALID";
    }
}


//Simple function to count digits, basically a loop that will go around chopping a number until its zero
//it counts each loop to get the digits
//Based on code by Acry (the func is identical) got it on Grepper
int countdigits(long number)
{
    int digits = 0;
    while (number > 0)
    {
        number = number / 10;
        digits++;
    }
    return digits;
}

//Simple exponation func
//Based on code by Numan from Bangladesh  got it on Grepper
long mypow(int base, int power)
{
    long result = 1;
    if (power > 0)
    {
        for (int i = 0; i < power; i++)
        {
            result = result * base;
        }
        return result;
    }
    else if (power == 0)
    {
        return 1;
    }
    //It does not deal with negative power since its a long I just set zero as the answer instead of dealing
    //with data type changing and stuff I wont need
    else
    {
        return 0;
    }
}